1. Field of the Invention
The present invention relates to a driver of a dot-matrix liquid-crystal display panel.
2. Description of the Related Art
FIG. 1 shows an arrangement of a conventional liquid-crystal display panel and its associated driver circuits. The arrangement of FIG. 1 comprises a dot-matrix liquid-crystal display panel 1; a common driver 2 for driving common (scan) electrodes X1 through Xn of liquid-crystal display panel 1; a segment driver 3 for driving segment (signal) electrodes Y1 through Ym; and a display control circuit 4 for controlling common driver 2 and segment driver 3. Waveforms for the FIG. 1 arrangement are illustrated in FIGS. 2A-2E and 3A-3E.
A prior art method for driving a liquid-crystal display panel 1, uses a time-division driving method. Referring to FIGS. 2A through 2E, a simple time-division driving method will be described taking, as an example, segment electrode Y1 and common electrodes X1-X4. FIG. 2A shows a horizontal synchronizing signal HD for determining the timing of application of a gradation signal to segment electrode Y1. A gradation signal is supplied to a segment electrode in synchronization with the fall of, for example, (FIG. 2A) an HD (horizontal synchronizing signal), i.e., in synchronization with the beginning of a selection period of a common electrode. FIGS. 2A to 3E show the timing of supply of signals, picking out Y1 from N segment electrodes and X1 to X4 from M common electrodes. Other segment electrodes also are supplied with gradation signals at the same timing. FIGS. 2B through 2 show the timing of application of common signals to common electrodes X1-X4 of common electrodes X1-Xn. In the figures, common electrodes X1-X4 are sequentially selected and gradation signals Y1,1-Y1,4 are applied to segment electrode Y1 in synchronization with selection of a common electrode. Pixels at the intersections of common electrodes X1-X4 and segment electrode Y1 are displayed in gradations (brightness) defined by signals Y1,1-Y1,4.
Such a driving method as shown in FIGS. 2A-2E has a disadvantage, however, that the duty ratio is lowered as the number of the common electrodes increases, thus lowering the contrast of displayed images. The lower limit of the presently practicable duty ratio is about 1/100. Incidentally, in liquid-crystal display panels used in small television receivers, the number of common electrodes is usually around 240 (the number of effective scanning lines per field). Where such a great number of common electrodes are driven simply on a time-division basis as described above, the duty ratio would become 1/240. This will result in images of such low-contrast as to have no practical use. FIGS. 3A to 3E show an improved prior art time-division driving method for solving the low duty cycle problem. According to this prior art method, a common signal is supplied to common electrodes X1-X4 in the time in which gradation signals Y1, is output, and a selection signal is supplied to common electrodes X2-X5 in the timing in which graduation signal Y1, 2 is output. The selection signal is thus supplied to four common electrodes in the same timing and, in other words, four common electrodes are selected (driven) in the same timing. This driving method shown in FIGS. 3A-3E is an improvement of the single time-division driving method and basically gradation signal Yp, s corresponds to common electrode Xs as in the simple time-division driving method.
Having as its aim the solving of the low duty ratio problem, this improved prior art method selects common electrodes adjacent to the common electrode desired to be selected simultaneous therewith. Referring to FIGS. 3A through 3E, this prior art improved time-division driving method will be described taking segment electrode Y1 and common electrodes X1-X4 by way of example.
FIG. 3A shows the timing of application of a gradation signal to segment electrode Y1, and FIGS. 3B through 3E show the timing of application of common signals to common electrodes X1-X4. When common electrode X4 is selected, for example, segment electrode Y1 is supplied with gradation signals "Y1,1", "Y1,2", "Y1,3", and "Y1,4". Hence, the display time of each pixel, is four times as long as that in the case of the simple time-division driving method shown in FIGS. 2A-2E. In a liquid-crystal display for a small television receiver with 240 scanning lines, the duty ratio thus becomes a practical level of 1/60 (4/240); however, the average pixel gradation at the intersection of segment electrode Y1 and common electrode X4 will become (Y1,1+Y1,2+Y1,3+Y1,4)/4. This is because the amount of light transmitted through a liquid-crystal cell depends on the effective value of the voltage applied. Therefore, the displayed gradation of each pixel differs from the true (desired) gradation, with the result that the resolution is lowered.